STYLE SWITCHER

CSS

IE9, IE10, and IE11 don't properly scale SVG files added with img tags when viewBox, width and height attributes are specified. View this codepen on the different browsers.
Image heights will not scale when the images are inside containers narrower than image widths. This can be resolved in 2 ways.

CSS Filters are a powerful tool that authors can use to achieve varying visual effects (sort of like Photoshop filters for the browser). The CSS filter property provides access to effects like blur or color shifting on an element’s rendering before the element is displayed. Filters are commonly used to adjust the rendering of an image, a background, or a border.

CSS variables, more accurately known as CSS custom properties, are landing in Chrome 49. CSS is going to be like a CSS pre-processor itself such as SASS, LESS in the future. CSS variables are now officially part of the CSS specification

Includes the .btn-default and .btn-info classes.
The .btn-secondary class isn't available in Bootstrap 3.

Introduced the .btn-secondary, .btn-light, and .btn-dark classes.
Dropped the .btn-default class.
Note that the .btn-info class was initially dropped in Bootstrap 4 but then reappeared again.

Outline Buttons

Not supported.

Introduced the .btn-outline-* classes for styling buttons with an outline color.
Introduced the .btn-*-outline classes for styling buttons with an outline color.
(The .btn-*-outline classes became .btn-outline-* in Alpha 3).

Button Sizes

The .btn-xs class is available.

Dropped the .btn-xs class (only .btn-sm and .btn-lg are available now).

Images

Responsive Images

Use .img-responsive class.

Use .img-fluid class.

Image Alignment

Use .pull-right, .pull-left, and .center-block helper classes.

Uses .m-x-auto instead of .center-block to align block-level images.
Can also use the various .pull-*-right and .pull-*-left responsive helper classes, as well as the .text-*-left, .text-*-center, and .text-*-right helper classes on the image's parent.
Can use the various .pull-*-none classes to disable floating.

Media Objects

Classes

Includes many different classes for media objects, including .media, .media-body.media-object, .media-heading, .media-right, .media-left, and .media-list and .media-body.

Uses just .media class. Margins can be applied using spacer utilities. Media objects are flexbox enabled in Bootstrap 4, so the various flexbox classes can also be applied (such as reordering, etc).

Can use the .nav-inline class to explicitly specify navs to be displayed inline.

Navbars

Colors

Limited (preset) color options. Supports inverse navbars but not the other classes.

New (preset) color options. Introduced the .navbar-light and the .navbar-dark classes, and allows the .bg-* classes to be used on navbars.

Navbar Alignment

Use .navbar-right, .navbar-left to align components within the navbar.

Use the various .pull-*-right and .pull-*-left responsive helper classes.Can also use the various .pull-*-none classes to disable floating.
Can either use spacing utilities such as .mr-auto, or any of the flexbox alignment utilities.

Navbar Forms

Add the .navbar-form class to forms within navbars.

Bootstrap 4 dropped the .navbar-form class. It's no longer necessary.

Fixed Navbars

Uses .navbar-fixed-top and .navbar-fixed-bottom to fix navbars to the top or bottom.

Uses .fixed-top and .fixed-bottom.

Pagination

Default Pagination

Only requires .pagination to be added (to the ul element that represents the list of pages).

Must also add .page-item to each li element and .page-link to each a element.

Pagers

Uses .previous and .next for aligning pagers.

Pagers have been dropped in Bootstrap 4 (Alpha 3).
Uses .pager-prev and .pager-next for aligning pagers.

Labels

Pill Labels

The .label-pill class is not available. However, Bootstrap 3 does have badges (which achieves a similar visual effect).

Labels have been replaced by badges in Bootstrap 4.
Badges can use the .badge-pill for rounded corners.

Tags

Supported?

No.
"Tags" are called "Labels" in Bootstrap 3 (i.e. they use the .label class).

Yes.The .tag class was introduced in Bootstrap 4 (Alpha 3) to replace the .label class.This was done to disambiguate from the label element.
Tags have now been renamed to "badges". These replace labels from Bootstrap 3.

Also requires .breadcrumb-item to be applied against all li elements that make up the breadcrumb.
Breadcrumbs can also be used outside of lists. For example, the .breadcrumb-item could be applied to a bunch of a elements that are wrapped in a .breadcrumb element.

The unset CSS keyword is the combination of the initial and inherit keywords. Like these two other CSS-wide keywords, it can be applied to any CSS property, including the CSS shorthand all. This keyword resets the property to its inherited value if it inherits from its parent or to its initial value if not. In other words, it behaves like the inherit keyword in the first case and like the initial keyword in the second case.